Chemical vapor deposition for an interior of a hollow article with high aspect ratio

ABSTRACT

A method and apparatus for plasma enhanced chemical vapor deposition to an interior region of a hollow, tubular, high aspect ratio workpiece are disclosed. A plurality of anodes are disposed in axially spaced apart arrangement, to the interior of the workpiece. A process gas is introduced into the region. A respective individualized DC or pulsed DC bias is applied to each of the anodes. The bias excites the process gas into a plasma. The workpiece is biased in a hollow cathode arrangement. Pressure is controlled in the interior region to maintain the plasma. An elongated support tube arranges the anodes, and receives a process gas tube. A current splitter provides a respective selected proportion of a total current to each anode. One or more notch diffusers or chamber diffusers may diffuse the process gas or a plasma moderating gas. Plasma impedance and distribution may be controlled using various means.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from prior U.S. provisional patentapplication Ser. No. 61/288,193 filed Dec. 18, 2009, prior U.S.provisional patent application Ser. No. 61/288,185 filed Dec. 18, 2009,prior U.S. provisional patent application Ser. No. 61/288,178 filed Dec.18, 2009, prior U.S. provisional patent application Ser. No. 61/288,172filed Dec. 18, 2009, and prior U.S. provisional patent application Ser.No. 61/370,659 filed Aug. 4, 2010.

TECHNICAL FIELD

The present invention relates generally to coating processes and inparticular to chemical vapor deposition (CVD) applied to an interiorsurface of a hollow article.

BACKGROUND

Plasma Enhanced Chemical Vapor Deposition (PECVD) methods for coatingexternal surfaces of a workpiece within a vacuum chamber are well known.The coating of interior surfaces of hollow workpieces, such as pipes,using PECVD techniques is less common, but has been described in U.S.Pat. No. 7,300,684 to Boardman et al., which utilizes a high depositionrate PECVD technique. The Boardman et al. method involves using theworkpiece itself as a vacuum chamber, coupling a gas supply to oneopening and a vacuum pump to another, and employing a voltage biasingsystem in which the negative terminal is attached to the pipe and thereturn anodes are located near the ends of the pipe but isolated fromthe pipe. The gas supply provides hydrocarbon precursors and the voltagebiasing system is used to generate a high density hollow cathode plasmaand to attract hydrocarbon ions to the surface, so as to form adiamond-like carbon (DLC) film on the interior surface of the pipe.Alternatively, non-hydrocarbon precursors can be utilized to formcoatings other than DLC.

As used herein, the term “hollow cathode effect” is an occurrence suchas in a tubular geometry with an axial anode, in which at least twocathode surfaces are positioned facing each other with a space betweenthe cathode surfaces and the anode, and biasing and pressure parametersare such that a large increase in current is achieved as compared to aconventional plasma glow. Such cathode surfaces can be coaxial internalwall surfaces in a pipe. The increase in current is due to the“oscillation motion” of fast (hot, accelerated) electrons between theopposite space charge sheaths, which enhances the excitation andionization rates in the plasma several orders higher than in theconventional glow discharge. The following definitions and descriptionsof the hollow cathode effect are contained in the publication entitled“STUDIES OF HOLLOW CATHODE DISCHARGES USING MASS SPECTOMETRY ANDELECTROSTATIC PROBE TECHNIQUES” by H. S. Maciel et al., 12^(th)International Congress on Plasma Physics, 25-29 Oct. 2004, Nice(France). Hollow cathode discharges are capable of generating denseplasma and have been used for development of high-rate, low-pressure,high-efficiency processing machines. The geometric feature of a hollowcathode discharge promotes oscillations of hot electrons inside thecathode, thereby enhancing ionization, ion bombardment of inner walls,and other subsequent processes. At the same time, the hollow cathodeexhibits plasma density one to two orders of magnitude higher than thatof conventional planar electrodes. “It is well known that the product(Pd), of the inter-cathode distance (d) by the pressure (P), is animportant parameter to describe the behavior of the HC discharge.Usually, the electron-atom inelastic collision rates are increased bythe decrease of the inter-cathode distance with a large effect on theplasma density and electron temperature. The effect of the gas pressureon the discharge properties is expected since the increase in thecollisionality by increasing the pressure tends to enhance the hollowcathode effects being possible to reach an optimized reducedinter-cathode distance (Pd).”

The system described in the Boardman et al. patent operates well for itsintended purpose. However, for relatively long and high aspect ratiopassageways, there are potential difficulties with maintaining plasmauniformity down the full axial length. As used herein, the “aspectratio” of a passageway within a pipe or other workpiece is defined asthe ratio of the length of the passageway to the cross sectionaldimension (typically, a diameter) of the passageway. In conventionalapproaches, a pipe or other tubular workpiece may be placed for externalcoating in a chamber in which dimensions are designed such that there islittle change in pressure throughout the chamber. However, when usingthe interior of the workpiece as the chamber, the dimensions of thechamber are defined by the intrinsic internal dimensions of theworkpiece. For high aspect ratio workpieces in which the hollow cathodeeffect is utilized, there is a weak plasma within the central region ofthe interior passageway of the workpiece, while the ends of thepassageway have an intense plasma. One possible explanation is that ahigh impedance is encountered by electrons leaving the center of theworkpiece (which is biased as the cathode) while a lower impedance isencountered by electrons leaving the ends. As a result, electron currentis shunted to the ends of the workpiece.

One possible solution is described in US Publication No. 2006/0196419 toTudhope et al., which is assigned to the assignee of the presentinvention. As described in this reference, the interior surface of aworkpiece can be coated in sections. Rather than having anodes attachedat the opposite ends of the workpiece, a pair of anodes are locatedwithin the workpiece at a distance from each other and aresystematically moved along the length of the workpiece. Thus, while theaspect ratio of the workpiece is not controllable, the aspect ratio ofthe section being coated is controlled. Another method relevant to theBoardman et al. patent is described in US Publication No. 2006/0198965.Rather than a continuous flow in one direction, the flow of gas issystematically reversed for the purpose of providing a more uniformcoating along the interior surface of the workpiece.

While the use of the hollow cathode effect is not described, otherapproaches that are of interest are described in U.K. Patent ApplicationNo. 2030180 A to Sheward. In one embodiment described in Sheward, apositively biased anode extends along the length of the interiorpassageway of a tube being internally coated. In an alternativeembodiment, the solid anode is replaced with an anode having a series ofholes through which relevant gas is released.

A concern with placing an anode wire along the axis of a high aspectratio passageway is that while a plasma may be maintained, the hollowcathode effect is easily lost and the deposition rate is lowered.Moreover, as the plasma impedance down a long workpiece can vary formany different reasons, including differences in pressure, gascomposition, distance between the electrodes, and incidental coating ofthe anode wire, plasma intensity is further reduced and/or “hot spots”develop as plasma concentrates at one or more still high conductivityregions of the anode.

Tubular structures with coated interior carrier surfaces are describedin U.S. Pat. No. 7,351,480 to Wei et al. Plasma immersion ion processingfor coating of hollow substrates is described in US Publication No.2008/0292806 A1.

Further improvements to the coating of high aspect ratio passageways aresought.

SUMMARY

A method, a system and an apparatus for plasma enhanced chemical vapordeposition (PECVD), using multiple anodes, are disclosed. Using PECVD, acoating is deposited to surfaces of an elongated interior region of ahollow workpiece.

Multiple anodes are inserted, in longitudinally spaced apartarrangement, along an elongated interior region of the workpiece. Aholder is dimensioned to distribute the anodes along the interior regionof the workpiece. The holder may have an elongated support tube whichfits in the interior region of the workpiece and arranges the anodesalong the interior of the support tube.

A process gas is introduced into the interior region of the workpiece.The holder has a process gas conduit which is connectable to deliver theprocess gas to the interior region of the workpiece. The support tube,positioning the anodes, may be operable to receive a process gas tubethat delivers the process gas.

A respective individualized DC or pulsed DC bias is applied to each ofthe anodes. This bias excites the process gas into a plasma, forapplying a coating to interior surfaces of the workpiece. The workpieceis biased as a cathode.

An electrical biasing circuit provides the individualized DC or pulsedDC bias to each anode. The electrical biasing circuit may make use of acurrent splitter connected such that each anode is provided a DC orpulsed DC current at a respective selected proportion of a totalcurrent.

Pressure is controlled inside the workpiece, so that the plasma ismaintained. A hollow cathode effect may be achieved in the plasma.

A chamber diffuser may be used to diffuse the process gas into theinterior of the workpiece. A plasma moderating gas may be introduced,and diffused into the interior of the workpiece using a notch diffuser.Further, plasma impedance and distribution may be controlled usingvarious means.

A chemical vapor deposition interior-coated apparatus has a hollowtubular substrate of high aspect ratio greater than or equal to aboutthirty to one, with a diamond-like or doped diamond-like coating layer.The diamond-like or doped diamond-like coating layer has a substantiallyuniform thickness greater than about twenty microns. The diamond-like ordoped diamond-like coating layer has a substantially uniform hardnessgreater than about nine gigapascals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a system using a holder having multiple anodesfor plasma enhanced chemical vapor deposition to an interior region of aworkpiece in accordance with an embodiment of the present invention.

FIG. 2 is a diagram of a biasing arrangement for the apparatus of FIG.1.

FIG. 3 is a diagram of an anode suitable for use in the apparatus ofFIG. 1.

FIG. 4 is an electrical schematic of a differential mode transformersuitable for use in the apparatus of FIG. 1.

FIG. 5 is an electrical schematic of a current splitter suitable for usein the apparatus of FIG. 1 and making use of the differential modetransformer of FIG. 4, for three anodes such as the anode shown in FIG.3.

FIG. 6 is an electrical schematic of a current splitter as adifferential mode transformer application for ten anodes such as theanode shown in FIG. 3.

FIG. 7 is a block diagram of an anode and splitter network suitable foruse in the apparatus of FIG. 1.

FIG. 8 is an electrical schematic of a variable output splitter networksuitable for use in the apparatus of FIG. 1 or in the splitter networkof FIG. 7.

FIG. 9 is a perspective view of a variation of the apparatus of FIG. 1,showing an anode support tube or lance inserted into an elongatedinterior of a workpiece.

FIG. 10 is a perspective view of a notch diffuser suitable for use inthe apparatus of FIG. 9.

FIG. 11 is a perspective view of a gas chamber diffuser suitable for usein the apparatus of FIG. 9.

FIG. 12 is a diagram of a zone-based cooling arrangement for promotingplasma uniformity, as a variation of the apparatus of FIG. 1.

FIG. 13 is a flow diagram of a method for plasma enhanced chemical vapordeposition to an interior region of a workpiece, which may be appliedusing the apparatus of FIG. 1, FIG. 9, FIG. 12 or another variation.

FIG. 14 is a cross-section view of layers deposited to a workpiece,using the method of FIG. 13.

DETAILED DESCRIPTION

With reference to FIG. 1, the present invention provides a system 100,an apparatus and a method for applying a plasma enhanced chemical vapordeposition coating to an elongated interior region of a hollow pipe,tube or similar structure, all called “workpiece”. Multiple anodes, eachof which is individually electrically biased, are disposed in aworkpiece, resulting in a more uniform coating of the interior region ofthe workpiece. Advantage is gained by the employment of the hollowcathode effect. It will, however, be appreciated that some of theaspects of the system and method described herein may be adapted with aplasma being generated that is not a hollow cathode type.

In accordance with the invention, a high deposition rate uniform coatingof a high aspect ratio interior passageway is enabled by providingsafeguards against “hot spots” along a lengthwise anode or anarrangement of multiple anodes within a workpiece being internallycoated. In an embodiment, a diamond-like coating (DLC) is applied to aworkpiece with an aspect ratio of greater than 30:1, and in a furtherembodiment a coating is applied to a workpiece with an aspect ratiogreater than 100:1. This range of embodiments defines “high aspectratio” workpieces, i.e., pipes, tubes and the like.

Hot spots are those areas along the anode length at which conditions areunintentionally more conducive to plasma development than other areasalong the anode length, so as to lead to inconsistent or incompletecoating along the length of the workpiece. The possible development ofhot spots can be controlled electrically with multiple anodes andcurrent splitting. It is also controlled mechanically with anode 20placement, an anode support tube 18, gas injection placement, gasdiffusers or multiple gases. For a high quality DLC coating, thetemperature measured on the outside of the workpiece should be less than600 F (about 315.5 degrees C.) and above 200 F (about 93 degrees C.) andpreferably below 500 F (about 260 degrees C.) and above 250 degrees F.(about 121 degrees C.).

For applications such as corrosion resistance, the coating applied to anarticle must be thick enough to prevent any corrosive material fromreaching the substrate. Also for abrasion or erosion applications, thecoating must be thick enough to prevent deformation of a soft substrate.Rapid deposition of coatings thicker than 10 microns, 20 microns, andgreater than 40 microns are observed. A deposition rate of greater than5 microns per hour or preferably greater than 8 microns per hour can beachieved.

To achieve high deposition rates with low pressure drop, precursors witha high number of carbon atoms per molecule are used as a process gas.For example, ethane has a higher deposition rate than methane, as eachion of ethane will deliver twice as much carbon as each ion of methane.Additionally, it has been found that double and triple bonded carbonmolecules provide higher deposition rates than single bonded carbonmolecules. For example, ethyne (acetylene) produces a higher rate thanethene, and ethene a higher rate than ethane. Suitable process gasesinclude, singly or in various combinations with other gases, methane,ethane, ethene, acetylene, silane, methylsilane, tetramethylgermaniumand tetramethylsilicon. Various hydrocarbons are suitable for use in oras a process gas.

Further, to achieve high deposition rates the highest current andhighest pulsed DC power duty cycle is used without exceeding thetemperature limits (550 degrees F.) to achieve a high quality DLCcoating and without causing arcing. For a high rate process, the highestpressure is used that achieves a high quality coating avoiding gas phasenucleation, which occurs at high pressures. In an embodiment, theaverage pressure is less than 400 mtorr but greater than 50 mtorr andthe current flux is between 2.4 mA/cm² and 9.5 mA/cm². In a furtherembodiment, the average pressure is between 150 mtorr and 250 mtorr andthe current flux is between 3.5 and 7 mA/cm², with the flux being equalto the average current per pulse divided by the surface area of theinterior of the workpiece 10. An example high quality DLC coating thatis deposited on the interior of a pipe using an embodiment of thedisclosed method, system and apparatus has hardness greater than 8 GPa(gigapascals) and preferably greater than 10 GPa, with a coefficient offriction less than 0.1, is impervious to 15% HCL at room temperature,and has thickness uniformity of less than +/−20% and preferably lessthan +/−15%.

As shown in FIG. 1, a mechanical approach to reducing the likelihood ofhot spots in a PECVD system 100 is to locate the anode 20 or anodes 20within an elongated internal anode support tube 18, which is then placedwithin the workpiece 10. The anode support tube 18 may be biased as acathode in order to promote a high deposition rate by maximizing thehollow cathode effect. Electrically, the bias control subsystem 14 maybe connected to establish both the workpiece 10 and the anode supporttube 18 as cathodes, as shown in FIG. 2. This establishes the cathodearrangement described above with reference to the hollow cathode effect.However, in the preferred applications the required “facing” cathodesfor the hollow cathode effect are defined by the opposite interior wallportions of the cylindrical workpiece 10 facing each other across thehollow interior space of the workpiece 10.

In order to provide the environment in which the hollow cathode effectis promoted uniformly down the length of the workpiece 10, theanode-covering or anode support tube 18 within the workpiece 10 isformed of a conductive material that is biased as a cathode, i.e. avoltage lower than the anode, while the workpiece 10 is also biased as acathode, perhaps at the same voltage or a nearby voltage.

Moreover, for embodiments in which a series of anodes 20 are utilized,the spacing between adjacent anodes 20 as compared to a diameter of ahollow tubular workpiece is less than a first spacing aspect ratio, butgreater than a second spacing aspect ratio. This first spacing aspectratio is approximately 15:1 and the second spacing aspect ratio isapproximately 1:1. This creates a hollow cathode condition, rather thana glow discharge condition. A ratio of spacing between anodes to theinternal diameter of the workpiece that is greater than about 20:1results in overheating of anodes and cooling and reduction or loss ofplasma. A ratio of spacing between anodes to the internal diameter ofthe workpiece that is less than about 1:1 can effectively short outneighboring anodes, resulting in loss of individual control of anodeswhen the anodes are too close.

In a preferred embodiment the anode-covering or anode support tube 18 isnot biased but allowed to float electrically. By minimizing the diameterof the anode support tube 18 and keeping the above-mentioned spacing, ithas been found that a high deposition rate process is maintained. Sincea smaller diameter anode-covering or anode support tube 18 provides bothless pressure drop and a higher deposition rate, it is important thatthe materials used to provide electrical and purge gas connection to theinterior anodes 20 be as small in diameter as possible, while providinggood electrical and thermal protection.

There are benefits to forming the anode support tube 18 of ceramic or aceramic-covered metal. A ceramic liner may be placed inside the anodesupport tube 18. For applications in which the internal tube or anodesupport tube 18 is not biased as a cathode and/or in applications inwhich the internal tube is still able to function as a cathode, theceramic coating may also be applied to the exterior of the tube. Duringthe process of coating the interior surface of the workpiece, thisanode-covering or anode support tube 18 will also be coated. Flaking ofthis incidental coating is less likely to occur with this use of aceramic, since ceramic has a better adhesion to coating material such asDLC. Alternately, a metal anode holder can have the surface slightlyroughened by a technique such as sand blasting, as this roughenedsurface will also provide better adhesion of the coating and will reducethe likelihood of flaking. Other coatings or surface treatments to theinternal tube could be used to directly increase the adhesion, or thesurface area, to promote adhesion.

FIG. 1 shows a conductive pipe, component, article or other workpiece 10connected in a PECVD system 100 that includes a gas supply subsystem 12and a bias control subsystem 14. The workpiece 10 is shown as a singlepiece, but may be an assembly of tubes or pipes. A coating precursorgas, such as methane or acetylene, may be provided by the gas supplysubsystem 12. This gas is used in implantation or coating steps thatwill be described below. Argon or another suitable inert gas may beprovided as a second gas to allow plasma “pre-cleaning” of the interiorsurface 15 of the pipe or other workpiece 10. During the implantation orcoating step, the inert gas may be mixed with the treatment gas.

The gas supply 12 and/or the pumping speed control valve are controlledto provide the pressure for establishing the hollow cathode effectplasma for a given diameter workpiece 10. While not shown in FIG. 1, avacuum subsystem is provided at the opposite end of the workpiece 10.Vacuum connections 37 are provided where needed, and these connectionscan also supply electrical isolation as appropriate. The vacuumconnections 37 may include end-caps, bands, O-rings or other sealingelements. In one embodiment, the system has gas introduced alternatelyfrom one end and then the other with pumping alternating from theopposite end. Additionally in this arrangement some percentage of thegas can be injected into the workpiece itself at one or more locationswithin the interior of the workpiece 10. In an arrangement for very highaspect ratios, gas is injected into the interior of workpiece 10 using atube with optimally spaced and sized holes that can be placed within theanode holder or anode support tube 18 or attached to it. The gas ispumped out of the pipe or other workpiece 10 using a pumping arrangementthat pumps equally from both ends of the workpiece 10 simultaneously.Both of these arrangements help to minimize the pressure drop down theinterior region of the workpiece 10, thus allowing more uniform coatingand higher flows with a resulting higher deposition rate.

The pressure settings should be such that the pressure within theinterior of the workpiece 10 establishes a condition in which theelectron mean free path is related to the distance between the interiorsurfaces 15 of the workpiece 10 and the exterior surface of the anodesupport tube 18 that houses a number of anodes 20 and 22 (in the casewhere the anode support tube 18 is powered as a cathode). In the casewhere the anode support tube 18 is floating electrically, this pressureshould be related to the inner diameter of the workpiece 10. While onlytwo anodes 20 and 22 are shown in FIG. 1, the uniformity of plasmaintensity is promoted if the anodes 20 extend along the length of theworkpiece 10 being internally coated. The anode support tube 18 is usedto provide support for the internal anodes 20 and 22 and in oneembodiment gas distribution injector(s). In one embodiment the anodesupport tube 18 is pre-stressed and kept under tension by pulling on thetube 18 from both ends, to prevent “sagging” due to gravity and thermalexpansion from plasma heating.

In one embodiment, the anode support tube 18 is an internal metal tubethat is used to hold a gas distribution injector in addition to being ananode holder, to allow the coating of a high aspect ratio hollowworkpiece 10. Holes 26 are placed along the length of the gas injectortube with the size and/or spacing of the holes being designed so thatgas flow into each section of the workpiece is relatively equal. The gasinjector tube can be placed within the anode support tube 18 or attachedto the outside. Holes 26 are also placed in the anode support tube 18such that the hollow cathode effect is established along the length ofthe workpiece. The anode support tube 18 has a series of holes 26 thatare spaced in order to allow electron access to the anode 20 or anodes.The distance between the holes 26 should be sufficiently small toprevent weak plasma (or “cold spots”) from forming between the holes 26.Cold spots will form in the center of high aspect ratio cathodes(particularly in the case of a long pipe, where the only anodes are atthe opposite openings), if the anodes are too remote from the center ofthe cathode. Electrons which are generated at the center of theworkpiece 10 are furthest from either anode and have the highestimpedance path to travel to reach the anodes. Thus, at a certaincritical aspect ratio of the workpiece 10, the plasma impedance from thecenter of the cathode-biased workpiece to the anodes will become toolarge and current will shunt to the ends of the workpiece. This isdemonstrated both by a cold temperature in the center and hightemperatures at the ends and by a thinner coating in the center and athicker coating at the ends. The typical maximum spacing aspect ratio ison the order of 15:1, although some of the described embodiments mayincrease or decrease this figure. Both the spacing between the anodesand the spacing between the holes 26 in the anode support tube 18 areused to control plasma impedance. The distance of the interior anode 20to the exterior hole 26 in the anode support tube 18 can also be used tocontrol plasma impedance. Additionally, the size of the holes 26 may beused to control impedance due to such factors as restricting orincreasing electron and argon flow. As shown in FIG. 1, the size of theholes 26 may be adjusted using movable sleeves 38. This provides severalbenefits including providing some control of the plasma, with a smallerhole increasing the plasma impedance due to less access of electrons tothe interior anode. These sleeves also provide a baffle for gas flowcoming from the inside of the tube 18, preventing turbulence that couldnegatively affect the coating on the inside of the workpiece 10.

With respect to the anode-covering or anode support tube 18, thediameter is minimized in order to increase the potential deposition rateby providing less blocking of the hollow cathode effect. If the interiorceramic or metal anode support tube 18 has a diameter that isunnecessarily large, more electrons will reside within the tube 18 andtherefore fewer electrons will be available for ionization-increasingcollisions. There are also a number of pneumatic considerations. Areduction in the diameter of the anode-covering or anode support tube 18within the workpiece 10 will also reduce the pressure drop as measuredalong the length of the workpiece 10. Additionally, if gas is injecteduniformly down the interior of the workpiece 10 from the spaced holes 26of the tube 18, the total pressure drop will be reduced and thelikelihood of significant reactive gas depletion similarly will bereduced. In an embodiment, a pressure drop of less than 500 mtorr isused and in a further embodiment the pressure drop is less than 250mtorr. A high deposition rate is important for a commercially viablecoating. The amount of gas required and the amount of ion currentrequired are both proportional to the surface area of the workpiece 10to be coated. Therefore, for a high deposition rate process, both highgas flows and high power supply currents are factors. High gas flowsincrease the pressure drop down the pipe or other workpiece 10 butreduce the residence time. High currents greatly increase the likelihoodof “hot spots” and arcing.

FIG. 1 illustrates the connection of the individual anodes 20 and 22. Inthis embodiment, each anode 20 and 22 may be separately controlled. Thisallows the use of differential mode transformers 30 (or currentsplitters) as will be described below when referring to FIGS. 4, 5, and6. Each anode line 32 and 34 from the bias control subsystem 14 may beprotected within an electrically insulating pipe 16 and 17 (e.g. ofceramic material). Such an electrically insulating pipe 16 and 17provides electrical isolation between the anodes 22 and 20 so that thecurrent splitters 30 can apply different voltages to each anode 20 and22, in addition to providing thermal protection. The insulating tube 16or 17 is also used to feed inert gas (e.g. argon) to the anode 22 or 20,as an anode gas. This has several advantages including maintaining ahigher pressure region around the anode 20 or 22, which preventsreactive gas from depositing on the anode 20 or 22, which will increasethe impedance of the anode in the case of a semi-insulating coating suchas DLC. Because argon is an easily ionized gas, it also provides a lowimpedance plasma around the anode when used as an anode gas. A quartzshield 28 with a small diameter hole 36 at the end can be used tomaintain a high pressure inert gas purge, using an anode gas, around theanode 20. The quartz shield 28 (or similar high temperature material) isused to provide thermal protection from the high intensity, hot plasmaemitted from the anode 20, preventing damage to neighboring anode 22tubes. Quartz sleeves are also used to protect any exposed joints in theceramic to prevent unwanted current flowing to the anode wire in thisarea.

In an alternate embodiment, anodized metal (e.g., anodized aluminum) canbe used as the electrically isolating tube, as this may provide a lowercost solution. This type of anode, which is anodized on the outerdiameter and not anodized on the internal diameter, has the advantage ofproviding high electrical isolation between the outer diameters of themultiple anodes, with good conductivity at the active area of the anode(where the non-anodized interior surface is exposed). Quartz, ceramic orother high temperature material is used to provide thermal protectionfor the high temperature region around the active anode. The use of asmall internal diameter tube provides a high pressure inert gas regionaround the active anode area by purging argon or other anode gas throughthe internal diameter of the anodized tube, without the need for ananode cover (e.g. quartz). In addition to the axially aligned anodes,there may be an anode at each end of the workpiece 10. These end anodesmay play a dual function of providing a sealed environment and ensuringthe proper electrical environment for establishing the hollow cathodeeffect.

With reference to FIG. 2, three connections are shown from the biascontrol subsystem 14. Connection lines 27 and 29 respectively bias theworkpiece 10 and the internal tube 18 as cathodes. Connection line 32biases the anode 20. The biasing of the components may be steady-statevoltages, but there are advantages to providing DC pulses. A negativepulsed bias may be used to (a) create the necessary plasma, (b) draw anionized reactive gas to the surface to be treated, (c) allow ionbombardment of the coating in order to improve coating properties, suchas density and stress levels, and (d) allow control of coatinguniformity by adjusting the duty cycle so as to permit replenishment ofthe source gas during the “off” portion of the cycle. An embodiment usesbipolar, dual frequency DC pulses. Current is pulsed at a high frequencybetween 10 kHz and 80 kHz, preferably between 10 kHz and 60 kHz andstill more preferably 20 to 25 kHz. The high-frequency pulsing is burstduring intervals at a low frequency less than about 20 hertz. The highfrequency pulsing is mainly used to control the quality of the coating,with a higher duty cycle resulting in a harder DLC. A duty cycle ofbetween 10% and 80% is used in one example, and a duty cycle of between40% and 60% is used in a further example. The low frequency duty cycle,controlling the duration of intervals during which the high-frequencypulsing is burst, is used mainly to control gas precursor depletion inthe case of high aspect ratio pipes. To prevent gaseous precursordepletion, the low frequency off time should be at least equal to theresidence time of the gas to travel through the pipe or other workpiece10. In order to achieve such a low frequency off time, the frequencyitself and/or the duty cycle may be adjusted. The total duty cyclemainly controls the heating and deposition rate.

In an example, a high aspect ratio workpiece consisting of a 30 footlong pipe with 3 inch internal diameter is supplied with an averagepressure of 200 mtorr with a flow rate of 700 sccm of gas and a lowfrequency of power pulsing of 0.5 Hz to 10 Hz. In a further example, alow frequency of power pulsing between 1 Hz and 3 Hz is used, with aduty cycle between 5% and 50%. In a still further example, a lowfrequency between 1 Hz and 3 Hz is used, with a duty cycle between 8%and 20%. Bipolar electrical pulsing can be used with the depositionpulse applying a comparatively large negative voltage to the workpiece10 (cathode) with respect to the anode(s), while for the discharge pulsea comparatively small positive voltage is applied to the workpiece 10with respect to the center anode(s) (in this case they are biased ascathodes). The purpose of the discharge pulse is to remove positivecharge which builds up on the resistive DLC, which if not removed cancause arcing to occur. In an embodiment of this invention, thedeposition voltage is between 400 V and 5 KV and in a further embodimentthe deposition voltage is between 500 V and 2 KV.

With reference to FIG. 3, one embodiment of the anode 20 is shown asdescribed in U.S. Patent Application Publication No. 2007/0262059 toBoardman, which is assigned to the assignee of the present invention. Anelectrode material 40 is formed as a coil of filament wire and arrangedso as to extend along a longitudinal axis, such that the open rings ofthe coil circle around the axis and create an interior zone 42.Surrounding the electrode material coil 40 is a shield 44 which may beformed of clear quartz, for example. The shield 44 defines a side wallwith a number of holes 46 that have positions selected to suit thedesired direction of plasma creation. In one embodiment, only one holeis placed at the top of the shield 44, with size sufficiently small tomaintain a higher pressure of inert gas inside the shield compared tooutside of the shield, so as to prevent reactive gas from coating theanode coil. Reactive gas is introduced through tube 52. These holes 46may be aligned with the holes within the tube 18 to control anodeimpedance. For example if the spacing between an anode 20 and theclosest hole in tube 18 is too small the plasma impedance will be verylow and this can cause “arcing” to one or more anode(s), if the spacingis too large the impedance will be very high, which requires highvoltage from the power supply and can exceed the limits of the supply.Alternatively, the functions of the shield may be performed by theinternal tube 18. Thus, referring briefly to FIG. 1, rather than anumber of anodes 20 and 22 contained within a single internal tube 18,the electrodes may be aligned in a manner in which each anode has ashield that functions as a segment of a discontinuous “interior tube.”

Again referring to FIG. 3, the anode gas (G) that passes through theholes 46 in the shield 44 and passes through the holes 26 within theanode support tube 18, is inert and provides a low impedancenon-reactive plasma around the anode 20. The electrode material 40 ispreferably a refractory metal such as tungsten, tantalum, or molybdenum.If the electrode material is tungsten, a suitable diameter is onebetween 0.1 mm and 1.0 mm, and in one example is 0.2 mm. The electrodematerial enters a base portion 48 of the anode 20 and is electricallycoupled to a terminal 50. This terminal is positioned at the outside ofthe base portion so as to allow connection to the anode line 32 from thebias control subsystem 14 of FIG. 1. The anode shown in FIG. 3 is anexample of the external anode. Internal anodes have similar components,but connections of the gas lines at the base portion would occur outsidethe workpiece, so as to conserve space.

A gas inlet 52 enables supply of gas through the base portion 48 andinto the interior of the shield 44, as indicated by arrows 54. While notshown in FIG. 1, each gas inlet 52 is connected to the anode gas supplyline 56 from the gas supply subsystem 12. In an alternative embodiment,a larger area anode, such as a conductive rod, is used as the activeanode. This has the advantage of reducing the current density andheating of the anode. In this case no shield or a shield with a largerhole size is used to provide full electron access to the anode surfaceand prevent heating.

With respect to FIGS. 4-8, electrical aspects relating to multiple anodesystems are shown. In one electrical control embodiment, more than oneanode is located within the passageway being coated. For example, aseries of anodes may be located along the axis of the passageway and theanodes may be separately biased. The spacing of such anodes may or maynot be uniformly distributed along the length of the workpiece. Even ifall of the anodes are merely connected to a fixed voltage supply, thevoltage drops across the short anodes will have less of an effect onplasma intensity than the voltage drop across a single anode that spansthe length of the passageway. However, additional benefits are achievedin a more complex embodiment in which a differential mode approach isused in establishing the currents to the various anodes. For example, adifferential mode transformer may provide currents to adjacent axiallyaligned anodes, such that changing current to one anode affects currentto the adjacent anode. Because of this interaction of currents todifferent sections along the length of the workpiece, the formation ofhot spots is retarded and the uniformity of coating is promoted.

FIGS. 4, 5, and 6 illustrate the use of differential mode transformersfor supplying current to sets of two anodes, three anodes, and tenanodes, respectively. By using the differential mode transformers, thecurrent for each section within the workpiece is similarly controlled.Anodes which are “wound against each other” provide compensation for thechanges in plasma impedance generated by the adjacent anodes. Thisreduces the likelihood of formation of significant “hot spots” and “coldspots.” That is, there is a greater uniformity of plasma along thelength of the workpiece and the resulting coating is more uniform. Ineach of the anode current splitters of FIGS. 4, 5, and 6, coaxial ortwisted-pair wiring may be used for the windings on a magnetic core.

With reference to FIG. 4, a two-way anode current splitter has thecurrent input (Iin) provided to opposite sides of the two windings oftransformer 62. Each resulting anode output line 64 and 66 receives onehalf of the current of the input 59. More significantly, a change ofimpedance of one of the anodes relative to the other anode will be atleast partially compensated as a consequence of the interaction of thecurrent pass. A difference in current flow between the two anodesfeeding into the splitter will induce a voltage across the splitter thatwill tend to force the current difference toward zero.

With reference to FIG. 5, a divide-by-three arrangement is shown inwhich each of three current output lines 68, 70, and 72 is connected toa different anode to provide one-third of the current from an input 74.But the arrangement is more than merely a division of input current.Rather, there is interaction of current paths to the anodes, since eachof three transformers 76, 78, and 80 has oppositely directed currentflows through the windings of the transformer.

With reference to FIG. 6, a further example of a current splitter usingdifferential mode transformers is shown. For situations in which aworkpiece has a particularly large aspect ratio, the number of anodesrequired for ensuring appropriately sized segments of the workpiece willbe large. For example, if each segment is one meter in length and theworkpiece is ten meters, the interconnection to the first five anodesmay be made through one end of the workpiece, while connections to theremaining five anodes may be made through the opposite end. This isrepresented in FIG. 6. A first current line 82 is associated with aseries of transformers with oppositely directed currents, as describedwith reference to FIGS. 4 and 5. Thus, five anode currents interact witheach other. Identically, a second current line 84 is associated withfive transformers and five interacting anode currents. The first fiveanode currents may be directed along lines which enter from one side ofa passageway while the remaining five anode lines enter from theopposite side.

With reference to FIG. 7, a splitter 700 is shown as built up fromfurther splitters in a network. In one embodiment the splitter networkis combined such that regions of the workpiece that tend to develop “hotspots”, and are believed to be associated with low plasma impedance,have a lower percentage of current directed to anodes in that area ofthe pipe. The splitter shown in FIG. 7 as a splitter network is suitablefor a 30 feet (9.14 meters) long pipe with an aspect ratio of 120:1,with current split to drive ten internal anodes and two external anodes.Note that two variable output divide-by-ten splitters 800 are used. Eachof these divide-by-ten splitter 800 boxes has two outputs (A and B) thatallow the percentage current to be varied in increments of 10% from 10%to 90% by adjusting control switches (i.e., A=10% and B=90%, A=20% andB=80%, A=30% and B=70%, etc.). In the example, each of the divide-by-tensplitters 800 is set to split a respective input current by 70% and 30%.Eight divide-by-two splitters 73 are further connected to the twodivide-by-ten splitters 800 so that the splitter network and splitter700 provides the various individualized respective connections 77 and 75and associated currents for the anodes to which the splitter 700 isconnected. Each anode thus receives a respective selected proportion ofthe current supplied by the high current DC pulser 71, to which thesplitter 700 is connected. Variations of the splitter 700 and furthersplitters and splitter networks may be devised.

In the embodiment shown in FIG. 7, the divide-by-ten splitter 800 boxesare combined with divide-by-two splitters 73 such that 11 amps ofcurrent are directed to each of the middle anodes 75, and 13 amps ofcurrent are directed to the end anodes 77. This data is typical butthese ratios may be changed during the process or for different processconditions. Significantly, it has been determined that the spacingbetween adjacent anodes should be less than a specified aspect ratio,which has been identified as 15:1, i.e. anodes should be spaced apart bya distance less than fifteen times a diameter of the passageway. Such aspacing aids in creating the hollow cathode effect, rather than a glowdischarge, although other spacings may be applied.

With reference to FIG. 8, an electrical schematic diagram shows thedivide-by-ten splitter 800 used in FIG. 7. The variable outputdivide-by-ten splitter 800 of FIG. 8 works in a manner related to thestatic five-way splitter in FIG. 6. Transformers X1 through X10 areconnected in differential mode. A respective relay, e.g. Relay 1 throughRelay 10, connects each of the ten splitter outputs (each output is 10%of the incoming current) to one pole of a relay or the other, with poleA going to a buss connected to output 1 and pole B going a bussconnected to the other output, namely output 2. The relays are activatedby ten switches on the outside of the box. If all ten switches areplaced in the A position then 100% of the input current goes to output 1and 0% goes to output 2. If two switches are in the A position, then 20%goes to output 1 and 80% to output 2, three switches=30% to output 1 and70% to output 2, etc. By making the splitter larger (more outputs) thedegree of current control becomes finer (a 100× splitter gives 1%increments, but this requires 100 splitters, and relays). Variations maybe devised having a lesser or greater number of outputs and fineness ofswitchable increments of current.

With respect to FIG. 9, an anode and gas delivery assembly 900 is shownas an implementation of the apparatus of FIG. 1. The anode and gasdelivery assembly 900 may be referred to as a lance, and is insertedinto an elongated interior region of a hollow article or workpiece 10.Multiple anodes 20 are arranged inside the elongated anode support tube18 of the anode and gas delivery assembly 900 or lance. A process gassupply line 63, tube or other conduit is attached to the outside of theanode support tube 18 of the lance. A plasma moderating gas supply line61, tube or other conduit is further attached to the outside of theanode support tube 18 of the lance. An anode gas supply line, tube orother conduit is routed inside the anode support tube 18 of the anodeand gas delivery assembly 900 or lance. The hollow article or workpiece10 receiving an internal coating is biased as a cathode, and the lanceor anode and gas delivery assembly 900 positions the multiple anodes 20and portions of the gas delivery conduits within the hollow article orworkpiece 10.

In the anode and gas delivery assembly 900 or lance example design shownin FIG. 9, larger anode slots 50 are placed close to the tip of theanode 20. These slots 50 are approximately 1.5 inch in length. Smallerholes 52 are placed between the anode slots 50 to allow some electronaccess between the anodes 20. This promotes a more uniform temperaturedistribution between the anodes 20, preventing the temperature fromdropping between adjacent anodes. Referring again to FIG. 9, this showsthe center section of the anode and gas delivery assembly 900 or lancedesign, with distance measured with reference to the center 51 of thelength of the lance for the hydrogen injector hole locations 54 andprocess gas injection locations 53. The hydrogen injector holes arelocated on the hydrogen gas line 61. When the hydrogen injector gas lineis installed to the anode support tube 18, these hydrogen injector holesare lined up with the hydrogen injector hole locations 54. Similarly,the process gas injection holes are located on the process gas line 63.When the process gas line is installed to the anode support tube 18,these process gas injection holes are lined up with the process gasinjection locations 53. Also shown are the gas chamber diffusers 58 andthe hydrogen notch type diffusers 60. The hydrogen gas injector isattached to the anode support tube 18 by slipping the hydrogen gassupply line 61 through support tubes 65 and hydrogen notch typediffusers 60 or other fittings on the top of the anode and gas deliveryassembly 900 or lance. The reactive gas injector is attached to theanode and gas delivery assembly 900 or lance by slipping the process gassupply lines 63 through support tubes 57 and gas chamber diffuser 58 orother fittings on the bottom of the anode and gas delivery assembly 900or lance.

A first set of anode and gas delivery assembly 900 or lance dimensionsis given in the example design below, and the dimensions are applicableto the apparatus of FIG. 9. In one embodiment, an anode and gas deliveryassembly 900, or lance, is dimensioned to fit into a 30 foot pipe havinga diameter of a few inches which is to be coated on the inside using aplasma enhanced chemical vapor deposition. The anode and gas deliveryassembly 900 includes an elongated anode support tube 18, which hasfittings for hydrogen or other plasma moderating gas, fittings forprocess gas and mountings for multiple anodes 20. The process gas supplyline 63 has process gas holes at specified locations 53, for injectingprocess gas. The hydrogen gas supply line 61 has hydrogen holes atspecified locations 54, for injecting hydrogen. Upon insertion of theprocess gas supply line 63 and hydrogen gas supply line 61 into theanode and gas delivery assembly 900 or lance, the holes in the processgas supply line 63 and hydrogen gas supply line 61 are aligned with thespecified locations, so that the respective gases may be dispensed intothe respective diffusers. Locations of the hydrogen holes 54, processgas holes 53 and anode tips 55 are symmetric about the center 51 of theanode support tube 18, i.e. pairs of each are located at the stateddistance from the center 51 of the anode support tube 18, one on eitherside of the center measured towards either respective end of the anodesupport tube 18. Hydrogen holes are located 2 inches, 15 inches, 38inches, 66 inches, 86 inches and 132 inches from the center 51. Processgas holes are located 1 inch, 23 inches, 48 inches, 76 inches, 103inches, 140 inches, 160 inches and 170 inches from the center 51. Anodetips are located 20 inches, 58 inches, 94 inches, 126 inches, and 154inches from the center 51.

One aspect in various embodiments is controlling gas chemistry down thelength of a long pipe. If precursor gas is introduced at one end of thepipe or other workpiece 10 and pumped out from the other end of theworkpiece 10 as in at least one of the variations, the gas will reactmore rapidly at the entry end forming a thicker coating at anintroduction end compared to the pump out end. Also as the reactant gasis consumed during the coating reaction, and byproducts are released,this difference in chemistry down the length of the pipe will causeadditional differences in plasma impedance down the length of the pipe.These differences in plasma impedance are a result of differingionization potentials as the chemistry varies along the length of thepipe. To correct this problem, at least one example uses a process gasinjector line of small diameter e.g. approximately ⅛ inch with processgas release holes of the proper size, and spacing between the holes, toprovide a uniform thickness coating down the length of the pipe. Anexample of a process gas injector design is described above, withreference to FIG. 9 and the dimensions applicable thereto. This designhas process gas release hole sizes of approximately 12 mils for theprocess gas injector, which can be attached to the anode and gasdelivery assembly 900 or lance. Since the process gas injector design issymmetrical the same hole pattern is repeated on the other side of thecenterline 51.

Another aspect of the invention is to provide a high quality coating(>10 GPa hardness, impervious to 15% HCL and hot NaCl) down the lengthof the long pipe or other workpiece 10. It has been found that even withuniform control of the plasma with multiple anodes using currentsplitters and even with uniform injection of reactive gas using a gasinjector, that some areas of the coating were of poor quality (<10 GPa)with the remainder of high quality. These areas of poor quality coatinghave multiple causes that are addressed by various embodiments.

The reactive gas that is introduced into the center of the pipe or otherworkpiece 10 has very low velocity. This low velocity of gas at thecenter of the pipe is a result of pumping out the gas from the ends ofthe hollow workpiece 10 in various embodiments, so that the lowest flowrate and thus lowest velocity is in the center of the workpiece 10. Asthe gas flows from the center toward the ends of the workpiece 10 andmore gas is added from the next gas injection holes, the gas velocitycontinues to increase. This low velocity gas in the center section ofthe workpiece 10 will more easily become depleted and is also moresensitive to turbulence caused by the introduction of high pressure,high velocity gas from the reactive or process gas injector holes. Thissituation is resolved in various embodiments by the introduction of aproperly distributed plasma moderating gas, which may be hydrogen or aninert gas. The hydrogen or other plasma moderating gas must bedistributed down the length of the pipe or other workpiece 10differently than the reactive gas, so a separate plasma moderating gasinjector is used. An example of the hydrogen injector design is shownabove with reference to FIG. 9 and dimensions applicable thereto. Theplasma moderating gas hole size used in this design is 10 mils for allholes except for the center hole which is 20 mils. It can be seen thathydrogen is preferentially delivered to the center section of the pipecompared to the ends, and this increases the velocity of the gas in thecenter section. Relative to the rate of production of a PECVD coating,hydrogen behaves as an inert gas and reduces the reaction rate of theprocess gas in locations where the hydrogen is applied. Both the highervelocity and lower reaction rate will reduce depletion of the reactivegas and prevent gas phase nucleation, resulting in a high qualitycoating. Hydrogen is chosen in one example over other inert gas as itcan diffuse into the coating and is thought to tie up dangling bonds,thus further improving the quality of the coating. Additionally arelatively high partial pressure of hydrogen in the gas phase will tendto suppress the formation of highly reactive double or triple bondedradicals and suppress polymerization reactions, all of which specieswould result in gas phase nucleation and poor quality coatings.

Another cause of poor quality coating is caused by high velocity gasjetting out of the small holes in the gas injector. This high velocitygas causes turbulence and disrupts the boundary layer on the pipesurface. A chamber diffuser 58, shown in FIG. 9 and discussed below withreference to FIG. 11, is used in one embodiment to uniformly releaselower velocity gas into the interior of the pipe close to the anode andgas delivery assembly 900 or lance, so that the boundary layer near theinterior surface 15 of the pipe or other workpiece 10 is not disturbed.A computational flow dynamics (CFD) simulation of the gas velocityexiting from a model of a chamber diffuser has shown that there islittle or no disruption of the boundary layer at the interior surface 15of the pipe where the coating is formed. Another aspect of at least oneembodiment is that the diffusers are designed to release gas only in thedirection of bulk gas flow (toward the pump out end of the pipe or otherworkpiece 10 and not toward the center) so that there is norecirculation of gas, which recirculation may cause poor qualitycoating.

Poor quality coating is also observed to occur near anode locations. Itis believed this poor quality coating is caused by the high fields andhigh energy electrons streaming into the anode slots 50 in the anode andgas delivery assembly 900 or lance, generating many radicals andreactive species and resulting in soft, thick and porous coatings. Asdescribed with reference to FIG. 9, this problem can be partiallyaddressed by controlling the size and distribution of the anode accessholes 50 in the anode and gas delivery assembly 900 or lance. In oneexample applicable to the anode and gas delivery assembly 900, theactive tip of the actual anode 20 is approximately 1 inch behind theanode slot 50 (i.e. the anode slot is at 19 inches from the center ormidpoint of a length of the anode support tube 18 and the anode tip isat 20 inches from this midpoint). If the anode tip is too close to theanode slot 50, electrical arcing takes place, to the detriment of thequality of the plasma and the coating being applied. If the anode tip istoo far back from the anode slot 50, the voltage will increase to anexcess DC voltage condition and the temperature will drop (e.g. if thetwo anodes symmetrically located at 21 inches towards each respectiveend from the center are pulled back to 25 inches, the center temperaturewill drop and the process voltage will increase). Too high a DC voltagecan damage the power supply. Proper placement of the anode tip relativeto the respective anode slot balances these two conditions, minimizingarcing and reducing the DC voltage that supports the current driving theplasma. An efficient plasma process is achieved with a greater currentand a lesser voltage, the greater current leading to a more rapiddeposition of a coating as a result of the larger number of carbon orother deposition product ions. To further resolve the above-describedproblem, in one embodiment an inert gas, for example hydrogen, isintroduced into the gas stream near anodes, which reduces the plasmaactivity around the anode and results in a high quality coating.

In one example, the reactive or process gas injector carries variouslower levels of inert gas mixed with the process gas. In one embodimentsilane, acetylene and methylsilane are used as reactive gases to form asilicon doped DLC coating. The silane is diluted with argon in the gasbottle, to reduce fire hazard, to a level of approximately 2% silane and98% argon.

Further, the inert gas injector may carry various lower levels ofreactive or process gas mixed with non-reactive, inert or plasmamoderating gas. In one embodiment small flow levels oftetramethylgermanium and/or tetramethylsilane are carried with higherlevels of hydrogen through the inert gas injector to form a germaniumand/or silicon doped DLC.

Various types of diffusers may be used to introduce various gases.Generally, a diffuser more uniformly releases a gas at a lower velocityas compared to gas released through a hole, a nozzle or a jet from a gasline. Gas may be initially released into a diffuser through a hole, anozzle or a jet from a gas line, then diffused into an interior regionof a workpiece through or by means of the diffuser.

With reference to FIG. 10, a notch diffuser 60 is shown. The notchdiffuser 60 is constructed from tubing 67, with a notch 69 cut orotherwise formed in the tubing 67. One or more examples of notchdiffuser 60 may be used for diffusing hydrogen or other plasmamoderating gas. One or more examples of notch diffuser 60 may be usedfor diffusing process gas, and/or a chamber diffuser may be used. To usethe notch diffuser 60, a gas supply line having a hole, nozzle, jet orother fitting for dispensing a gas is inserted into one end of the notchdiffuser 60, until the hole or other fitting is at the specifiedlocation. Locating the hole inside of the tubing 67 but not within thenotch 69 region results in a more even diffusion of the gas. In FIG. 9,a hydrogen hole is located between two notches 69 in the notch diffuser60. Gas exits at a high velocity from the holes in the hydrogen line andexits the notch diffuser 60 at the notches 69. As a result of thenotches 69 facing towards the anode support tube 18 and away from theinternal surfaces 15 of the workpiece 10 where the high quality coatingis being formed, the gas is diffused closer to the anode support tube 18and away from the coating forming region on the workpiece 10.

With reference to FIG. 11, a gas chamber diffuser 58 is shown. Thechamber diffuser 58 provides a more even diffusion of a gas than doesthe notch diffuser 60, and is utilized for diffusing the process gas.The chamber diffuser 58 has a chamber member 112 and a diffuser member114. In the example shown, the chamber diffuser 58 is constructed of anend ring 118 and a diffuser member 114, both of which clamp around theanode support tube 18 of the lance. An outer tube 116 (shown in dashedlines) fastens to the end ring 118 with a fastener 120. The outer tube116, the end ring 118, the diffuser member 114 and the anode supporttube 18 form the walls of the chamber or the chamber member 112. Gassupply support tubes 122 and 124 may be attached to the anode supporttube 18 or to the chamber diffuser 58. Process gas line 126 is routedthrough the appropriate gas supply support tube 124 and positioned sothat a process gas hole (not shown) aligns within the chamber member112. Gas from the process gas hole is emitted into the chamber of thechamber member 112 and diffuses outwardly from the chamber member 112through one or more through holes 128 fluidly connecting the chambermember 112 to the interior region of the workpiece. A hydrogen gassupply line 130 is routed through the chamber diffuser 58, but does nothave holes emitting hydrogen gas into the chamber member 112, when thechamber diffuser 58 is used solely for diffusing process gas. In theexample shown in FIG. 11, the end ring 118 does not have through holesto the chamber member, and blocks gas from diffusing through the endring 118. Further, in the example, the diffuser member 114 is recessedwithin the outer tube 116. Variations may be devised.

In one example, the chamber diffuser 58 is implemented as a clamshellthat clamps around the anode support tube 18. The process gas line 126slides through holes in the end ring 118 and the diffuser member 114.Gas exits through small holes in the gas line at high velocity andexpands into the diffuser chamber, then exits the larger holes 128 atthe bottom of the diffuser member 114 at lower velocity. The chamberdiffuser 58 lowers the velocity of the gas and directs the gas in thesame direction as the background bulk flow in the interior region of theworkpiece 10. In so doing, high velocity gas is kept near the anodesupport tube 18 and away from the interior surfaces 15 of the workpiece10.

With reference to FIGS. 9-11 the orientation of the workpiece 10 and thegas diffusers 58 and 60 relative to the workpiece 10 affects thedistribution of the plasma and the quality of the resultant coating onthe inside of the workpiece 10. Absent the gas diffusers, process gashas been observed gathering in the upper half of the hollow interior ofthe workpiece 10, possibly due to a buoyancy effect occurring even atlow pressures needed for a plasma using the hollow cathode effect. Ahorizontal orientation of the workpiece 10, or the elongated interiorregion of the workpiece to which the coating will be applied, is aconsideration for evenness of process gas and plasma distribution.Having holes on a lower half of a process gas diffuser, such as the gaschamber diffuser 58, introduces the process gas predominantly in a lowerhalf of the interior region of the workpiece 10, producing a more evendistribution of the plasma as compared to introducing the process gasevenly in the upper and lower half of the interior region and allowingthe process gas to gather in the upper half. Introducing hydrogen orother plasma moderating gas predominantly in the upper half of theinterior region of the workpiece 10 produces a more even distribution ofthe plasma as compared to not introducing a plasma moderating gas orintroducing a plasma moderating gas evenly in the upper and lower halvesof the interior region of the workpiece. These conditions are achievedby having a majority or all of a plurality of through holes 128 in thechamber diffuser 58 or other diffuser in the lower half of the diffusermember 114 when a chamber diffuser 58 or other diffuser is used for theprocess gas, and by having the notch diffuser 60 or other diffuser forthe plasma moderating gas positioned on the upper half of the anodesupport tube 18. Introducing the plasma moderating gas primarily in amid-length region of the interior region of the workpiece furtherprovides for evenness of process gas and plasma distribution, and can beaccomplished with suitable distribution of diffusers for the plasmamoderating gas. In a variation, the process gas is introduced into afirst region, and the plasma moderating gas is introduced into a secondregion. The first and second regions are within the interior region ofthe workpiece and oppose each other relative to the lengthwise axis ofthe workpiece.

In one example, applicable to FIGS. 9-11, process gas is introduced atthe center of the interior of the workpiece 10 through a notch diffuser60. The notch diffuser 60 is mounted to the bottom of the anode supporttube 18 at the center of the anode support tube 18. Further, process gasis introduced at one or more symmetric locations to either side of thecenter through symmetrically mounted chamber diffusers 58. The chamberdiffusers 58 are mounted to the anode support tube 18 at specifieddistance from the center of the anode support tube. A plasma moderatinggas is introduced at one or more symmetric locations to either side ofthe center through symmetrically mounted notch diffusers 60. The notchdiffusers are mounted to the top of the anode support tube 18 atspecified distance from the center of the anode support tube. Anodes 20are mounted in spaced apart arrangement along the interior of the anodesupport tube 18. Various holes are positioned at specified locations.

With reference to FIG. 12, an apparatus and a method for improvedcontrol of plasma uniformity based on workpiece temperature control aredescribed. For many coatings that may be applied to the interior of theworkpiece 10, plasma impedance within the active zone may be determinedby the temperature at the external surface of the workpiece 10. Thetemperature of the workpiece 10 increases primarily due to the plasmaion bombardment on the inner surface. Higher plasma intensity producesmore ion bombardment and accordingly larger increases in the externallymeasured temperature of the workpiece 10. Therefore, the plasmaintensity profile within the workpiece 10 is similar to its externaltemperature profile. For example, where the workpiece 10 is a conductivemember and the applied coating is DLC, changes in the plasma intensitysignificantly affect the exterior temperature of the workpiece 10. Theinverse is also true. That is, by cooling the exterior surface of theworkpiece 10, the plasma intensity is affected, as is the plasmauniformity.

FIG. 12 illustrates an approach to controlling the formation of hotspots, by controlling the axial temperature profile during the coatingprocess. Thermal control of potential hot spots may be provided bymonitoring the temperature at the different sections along the workpiece10 and providing appropriate cooling. For example, where the workpiece10 is an electrically conductive pipe and the interior of the pipe isbeing coated with a semiconductor coating, such as DLC, the temperatureof individual sections can be easily monitored and controlled along theexterior of the pipe because such a coating becomes more conductive asits temperature increases. For example, a temperature sensor 86 and acooling device, such as a fan 88, may be located at each section of 0.3meters. Since plasma impedance is indicated by the temperature at thesection, temperature control promotes coating uniformity. Optionally, bymerely ensuring cooling at the ends of the pipe, the impedance of thecoating at the ends will increase, forcing current and plasma to thecenter of the pipe.

As described above, each segment of the workpiece 10 may be associatedwith a different anode 20, 22, and 24. Along the workpiece is positioneda linear array of temperature sensors 86 and fans 88. In FIG. 12, eachsegment is associated with a temperature sensor 86 and a fan 88. If thetemperature detected by one sensor 86 is significantly greater than thatof the other sensors 86, the associated fan 88 may be used to controlthe plasma intensity at the hot spot. While FIG. 12 shows temperaturecontrol in segments that coincide with the segments defined by theanodes 20, 22 and 24, this is not a requirement. For example, there maybe three temperature sensors 86 for each one-meter segment defined bythe positioning of the anodes 20, 22 and 24. In one example, thetemperature sensors 86 are non-contact devices, such as IR devices,having outputs which are used to control operation of the associated fan88.

Further, the temperature can be measured by an IR temperature measuringdevice moving along a track parallel to the workpiece. Using theabove-described temperature measurement or another technique readilydevised, such temperature measurement device or devices send output to acontrol unit e.g. a programmed computer control unit which controls theoperation of a fan 88 or group of fans. If the temperature detected byany sensor 86, or at any location, is greater than its setpoint, then afan 88 or group of fans is activated as determined by the temperaturecontrol algorithm in the computer programming. The temperature sensorsare positioned uniformly along one side of the workpiece 10, or inanother distribution, in sufficient number to accurately reflect thetemperature profile of the workpiece 10. In one example, the temperaturesensors 86 are positioned in the horizontal plane along one side of theworkpiece 10. The fans 88 may be positioned below the workpiece 10. Thevertical positioning of the fans 88 is determined primarily by thevolumetric output of the fan 88, the fan diameter, and thermal radiationfrom the workpiece 10. During the process set-up step, the non-cooledheating rate profile of the workpiece 10 is observed with the fixedarray of temperature sensors 86 or using a handheld IR temperaturemeasuring device. The axial position and quantity of fans 88 aredetermined from this initial profile and adjusted so that thetemperature profile of the workpiece 10 can be reliably controlled.Thus, zone-temperature control is enabled.

Still further variations and improvements to plasma enhanced chemicalvapor deposition systems and methods are herein presented. Thesevariations may make use of portions or entireties of the systems,methods and apparatuses of FIGS. 1-13.

Referring back to FIG. 2, while the workpiece 10 and the anode supporttube 18 are shown as being connected to have the same voltage, there maybe benefits to providing differing cathode voltages to the workpiece 10and the anode support tube 18. The bias to the anode support tube 18 maybe controlled using a separate power supply or using resistors betweenthe inner and outer cathodes. Another possibility is to provide adifferential mode transformer to establish the cathode biases on theanode support tube 18 and the workpiece 10. The bias to the anodesupport tube 18 may be tailored to increase the deposition rate whileminimizing the power requirements, heating, and gas flow requirements.

The pressure (P) within the space between the workpiece 10 and theinternal tube 18 may also be tailored. The pressure may be set tooptimize the hollow cathode scaling with P*d remaining constant, where dis the distance between the workpiece 10 and the anode support tube 18.This tailoring creates a stronger hollow cathode, since there is noelectrically floating component blocking the hollow cathode effectelectrons which provide the ionization-enhancing collisions.

In one embodiment, a single internal central anode 20 is used. Incomparison to an approach using multiple small separately-controlledanodes 20, this approach has the advantages of reduced arcing due to thelarge anode surface area, smaller cross-section resulting in lesspressure drop, and simplicity of fabrication. A conductive rod or tubeis inserted down the center of the workpiece 10 and biased as a centralanode. The plasma impedance or uniformity down the length of theworkpiece has limited control by current splitters, since there is onlyone internal anode (compared to the case of multiple-internal anodes),although there is some degree of control with respect to the relativeamount of current delivered to the external anodes and the singleinternal anode. In this case, plasma uniformity is also controlled bycovering portions of the central anode tube with electrical insulators,such as ceramic or glass. This results in cooling of the plasma in theseareas. Plasma impedance or uniformity down the length can be adjusted byusing a non-uniform spacing of these covered areas down the length ofthe internal anode. Inert and reactive gas can be delivered to theworkpiece through tubes that run down the interior of the rod. Holes areplaced in the gas tubes and in the outer anode to deliver the gas whereneeded. In one embodiment, separate chambers are formed within the anodetube using spacers, with non-reactive gas (e.g. argon) delivered to somechamber(s) and reactive gas delivered to other chamber(s). The relativeflow of inert gas (e.g. argon) compared to reactive gas flow and thelocations where these gases are delivered can also be used to controlplasma impedance. In the case of bipolar pulsing, where a small negativevoltage is applied to the anode to dissipate positive charge build-up onthe semi-insulating coating, one of the internal tubes inside the anodeis a conductor and diodes are used to direct the negative anode pulse tothe internal tube, with this chamber of the anode tube being purged withinert gas. The positive anode pulse is delivered to the outside anodetube. Since a negative bias is required to make a hard, adherent DLC,this prevents the working anode (the positive pulse external tube) frombeing coated with DLC. In another embodiment, RF bias can be applied toone of the conductive tubes within the anode tube. Additionally oralternatively, cooling water can be feed through the tube.

The uniformity of the plasma impedance down the length of the work pieceis controlled by (1) the relative current split between the centralanode and external anodes located at each end and (2) the centralanode-to-cathode impedance.

Current provided only to the external anodes at both ends of theworkpiece 10 produces a plasma intensity (and temperature) profilegreatest at the ends, but decaying to non-useful levels at a distance ofabout 8 to 12 workpiece diameters away from the ends of the workpiece10. Conversely, current provided only to the internal anode produces aplasma intensity profile greatest at the center and decaying tonon-useful levels at a distance of about 8 to 12 workpiece diametersfrom both ends. Splitting the current between the external end anodesand the central anode produces an axial plasma intensity profile that isa combination of the two extreme profiles described above. In this case,the plasma intensity profile looks like a “W” with the intensity at theends being greater or less than the center depending on the relativedistribution of the current. Thus, tailoring of the profile toaccommodate a particular application is possible.

The central anode-to-cathode impedance is controlled by the anodeavailability, the characteristics of the plasma, and the coatingresistance. The anode availability at any location along the centralanode can be reduced by covering it with an electrical insulator, suchas ceramic or glass. The plasma impedance is influenced by many factorsincluding temperature, pressure, composition and electron density. Thecoating resistance is a function of temperature and typically increaseswith added thickness. For many coatings, including DLC, the resistancedecreases with increases in temperature.

An axially uniform plasma and temperature profile is desirable, since itproduces a uniform coating. The relative distribution of current betweenthe central anodes and the external end anodes depends on the aspectratio of the workpiece and where more or less coating is desired duringthe coating process. The distributed current “W” profile is made uniformas follows: (1) The middle hot spot (center of the “W”) is reduced bydecreasing the available area of the central anode at the center. Thearea is reduced by covering it with an electrical insulator, such asceramic or glass. This will result in decreasing the plasma intensity atthe center. If desired, other areas can be covered for the same effect.(2) The regions of lower plasma intensity (bottom points of the “W”) areincreased by adding external thermal insulation around the exterior ofthe workpiece. This reduces the rate of heat loss relative to otherpositions along on the work piece. (3) During operation, temperaturecontrol by continuous removal of heat is provided via forced-convectionzone cooling along the length of the workpiece. Controlling the axialtemperature profile adjusts the internal coating resistance, resultingin a uniform plasma intensity profile. The system of forced-convectionzone cooling is described below.

For aspect ratios of 40 or less, it is sufficient for the central anodeto be a small fluid-cooled tube under tension. For longer aspect ratios,the central anode is a larger diameter outer tube where inert andreactive gas can be delivered to the workpiece through smaller tubesthat run down the interior of the central anode. Holes are placed alongone of the smaller gas tubes and correspondingly in the larger outeranode tube to deliver the gas where needed. In one embodiment, separatechambers are formed within the outer anode tube using spacers, withnon-reactive gas (e.g. argon) delivered to some chamber(s) and reactivegas delivered to other chamber(s). The relative flow of inert gas (i.e.,argon) compared to reactive flow and the locations where these gases aredelivered can also be used to control plasma impedance.

In the case of bipolar pulsing, a small negative voltage is applied tothe anode to dissipate positive charge build-up on the semi-insulatingcoating. Without this charge dissipation, the rate of coating growthdecreases and arcing can occur. This small negative voltage on the anodemakes it act briefly as a “reverse cathode”. The negative bias promoteshard, adherent and insulating DLC to form on the anode, causing theanode it to lose effectiveness. The use of gas-purged external anodesmitigates this concern at the workpiece ends.

One option to protect the central anode tube from the negative biasduring the reverse pulse is to block this pulse with a diode. Only thepositive anode pulse is delivered to the central anode tube. This willresult in “non-energetic” coating deposition on the central anode. Thistype of deposition is electrically conductive at the elevated processtemperature. Towards the end of the coating process, where chargedissipation is more important, the diode is removed allowing the“forward anode” to also act as the “reverse cathode”. Since there is aconductive sub-coating on the anode before this point, the DLCdeposition formed after this point will be of poor quality and stillmostly conductive. After a period of time, it will becomenon-conductive. Sufficient charge dissipation occurs, allowing thecoating process to continue until that point in time is reached. Thispoint in time is characterized by an increase in the voltage required tomaintain the same current and arcing.

Another option is to allow the internal tube(s) inside the outer centralanode tube to also serve as a conductor. Only the positive anode pulseis delivered to the outer central anode tube, the reverse pulse beingblocked by diodes. Other diodes are used to direct the reverse pulse tospecific locations along the internal tube(s) which serve as a “reversecathode”. At these specific locations are chambers purged with inertgas.

In another embodiment, a small amount of RF energy can be applied alongthe outer central tube or along the interior conductive tubes withdifferent configurations of feed and return possible. The RF energy canhave beneficial effects by keeping plasma present even during offperiods in pulsed DC operation. This can prevent high strike voltagesduring pulse DC operation.

Additionally, cooling or heating fluid (e.g., water, steam, oil) can befeed through an interior tube for temperature control and to facilitateevaporation of liquid precursors.

The central anode tube is centrally positioned in the workpiece byplacing it under tension. Additionally, the central anode tube can bepre-stressed with a shape that when placed horizontally causes it tostraighten under the influence of gravitational force.

Additionally, to increase the stability of the horizontal alignment, thecentral tube under tension may have a slight applied torque at each end.The applied torque to the ends can be either clockwise orcounter-clockwise but must be opposed (i.e., the same when viewed fromeach end).

With respect to FIG. 13, a method 99 is described for plasma enhancedchemical vapor deposition of a coating to an elongated interior regionof a workpiece, such as a tube or a pipe or other hollow workpiece. Themethod 99 may be applied using the multiple anode apparatus of FIG. 1, 9or 12 or another variation as described or devised by a person skilledin the art based upon the teachings herein disclosed. The method 99 isapplicable to a workpiece with an aspect ratio greater than or equal to5:1 and to a workpiece with an aspect ratio greater than or equal to20:1, including workpieces with an aspect ratio of equal to 20:1, 30:1,64:1, 100:1 and 120:1. The method 99 is applicable to and scalable toaspect ratios of 150:1 or higher.

A tube, pipe or other hollow workpiece is prepared for plasma enhancedchemical vapor deposition of a coating to an elongated interior regionof the workpiece. Such preparation may include inspecting, cleaning,installation of vacuum and pressure fittings, bracing the workpiece,attaching the workpiece to an apparatus, orienting the workpiece (e.g.horizontally) and so on.

Multiple anodes are inserted 90 into the interior region of theworkpiece. The anodes are in spaced apart arrangement along theelongated interior region of the workpiece. The anodes may be insertedand/or arranged within the interior region of the workpiece at specifiedspacings or locations and using an anode support tube such as describedabove.

A process gas is introduced 92 into the interior region of theworkpiece. The process gas may be introduced via a process gas line andvia one or more diffusers such as described above.

A respective individualized DC or pulsed DC bias is applied 96 to eachof the anodes. The bias excites the process gas into a plasma. Theworkpiece is biased 94 as a cathode. Various circuits including currentsplitters, differential mode transformers, switches and connections suchas described above may be used in biasing the anodes and the cathode.

The pressure in the interior region of the workpiece is controlled 98 toachieve a hollow cathode effect in the plasma. Various techniques suchas described above may be used in controlling the pressure and inachieving a hollow cathode effect in the plasma.

Various techniques, systems, subsystems and devices as described abovemay be applied in order to control the plasma impedance, plasmadistribution, plasma uniformity and aspects of the resultant PECVDcoating produced on the interior of the workpiece.

With reference to FIG. 14, an embodiment of the method and the apparatusis used to produce articles including pipes with layered diamond-likecoatings deposited onto the internal surfaces 1404 of the pipes 1402.The pipes are 10 feet long and have 3 inch diameters, for an aspectratio of 40. The built-up coating on these pipes has four layers, eachof which is applied as a coating. A first layer 1406 includes a thinsilicon adhesion layer applied to the claimed internal surface 1404 ofthe pipe 1402. A second layer 1408 includes a blended silicon andgermanium adhesion layer applied to the first layer. A third layer 1410includes a silicon and germanium doped diamond-like coating layer withan increasing carbon content through the layer, applied to the secondlayer. A fourth layer 1412 includes a diamond-like “cap” layer appliedto the third layer. Measurements show a coating thickness ofapproximately 22 microns, a hardness of approximately 11-14 GPa and acoefficient of friction of 0.02-0.09.

In one example, a multilayer coating is produced on interior surfaces ofan elongated hollow workpiece made of steel, using an embodiment of themethod and the apparatus. In order to adhere a diamond-like coating, anadhesive layer is applied to the cleaned steel interior surface of theworkpiece. The adhesive layer is relatively pure silicon. The high ionbombardment creates a mixing of steel and silicon, such that of anapproximately 1000 angstroms thick layer, approximately 10% or 20% is amixed layer of silicon bonded with steel. After the adhesive layer, ablend layer is applied. As the blend layer is applied, the siliconcontent is slowly reduced by lowering the silane from 100% to 0%, andthe carbon content is slowly increased by raising the acetylene from 0%to 100%. This blend layer compensates for a mismatch in propertiesbetween pure silicon layers and pure carbon layers, and preventsdelamination. After the blend layer, as a topmost layer, a pure carbondiamond-like coating layer is applied as a “cap” layer.

A product is produced as an apparatus, using a disclosed method and adisclosed apparatus. A hollow tubular substrate of high aspect ratiogreater than or equal to about thirty to one has a diamond-like or dopeddiamond-like layer. The layer has a substantially uniform thicknessgreater than about twenty microns, and a substantially uniform hardnessgreater than about nine giga pascals. In a first variation, thediamond-like or doped diamond-like coating layer has a silicon orgermanium dopant.

In a second variation, the hollow tubular substrate has a steel interiorsurface, to which an adhesion layer is bonded. The adhesion layer isbetween the substrate and the diamond-like or doped diamond-like coatinglayer. The adhesion layer includes a mixed layer of silicon bonded withsteel. The adhesion layer is bonded by the mixed layer to the steelinterior surface of the hollow tubular substrate.

In a third variation, a blend layer is deposited between the adhesionlayer and the diamond-like or doped diamond-like coating layer. Theblend layer has a silicon content decreasing from greater than about 90%at the adhesion layer to less than about 10% at the diamond-like ordoped diamond-like coating layer. The blend layer has a carbon contentincreasing from less than about 10% at the adhesion layer to greaterthan about 90% at the diamond-like or doped diamond-like coating layer.

In a fourth variation, the diamond-like or doped diamond-like coatinglayer has a coefficient of friction of approximately 0.02 to 0.09.

In further examples, a pure silicon adhesion layer is applied, followedby a diamond-like coating with a low percentage of silicon i.e. a lowsilicon to carbon ratio. The low silicon to carbon ratio may be lessthan or approximately 1%. Further variations may be devised, with orwithout a cap layer.

What is claimed is:
 1. A method for plasma enhanced chemical vapordeposition (CVD) to an interior surface of a hollow tubular high aspectratio workpiece, the method comprising simultaneously coating the entireinterior surface as a single section by: providing at least three anodesin axially spaced apart arrangement along an entire lengthwise axis ofan elongated interior of a hollow tubular workpiece of high aspect ratiogreater than or equal to about thirty to one; providing an elongatedsupport tube dimensioned to reside in the elongated interior of theworkpiece along its entire length and arranging the anodes in spacedapart relation inside of the elongated support tube, wherein a ratio ofa spacing between anodes to an interior diameter of the hollow tubularworkpiece is between about twenty to one and about one to one; theworkpiece being biased as a cathode and the elongated support tube isallowed to float electrically or be biased as a cathode so to form aplasma region in the interior of the workpiece between the support tubeand workpiece, the support tube having a set of holes therein on allsides situated in proximity to the anodes within the support tube so asto allow electron flow between anodes and the plasma region; introducinga CVD process gas into the entire plasma region in the interior of theworkpiece; applying a respective individualized DC or pulsed DC bias toeach of the plurality of anodes to excite the process gas into a plasma;and controlling a pressure of the process gas in the interior of theworkpiece to maintain a plasma.
 2. The method of claim 1 whereinmaintaining a plasma includes maintaining a hollow cathode effect in theplasma.
 3. The method of claim 1 wherein introducing a process gas intothe interior region of the workpiece includes routing the process gasthrough one or more gas diffusers.
 4. The method of claim 1 wherein theinterior region of the workpiece is oriented horizontally and theprocess gas is introduced predominantly in a lower half of the interiorregion of the workpiece.
 5. The method of claim 1 further comprisingintroducing an anode gas in a vicinity of at least one of the pluralityof anodes.
 6. The method of claim 1 further comprising introducing aplasma moderating gas into the interior region of the workpiece.
 7. Themethod of claim 6 wherein the process gas is introduced into a firstregion and the plasma moderating gas is introduced into a second region,the first and second regions being within the interior region of theworkpiece and opposing each other relative to the lengthwise axis. 8.The method of claim 6 wherein the plasma moderating gas is introducedpredominantly in a mid-length region of the interior region of theworkpiece.
 9. The method of claim 1 wherein the CVD process gas includesconstituents of a diamond-like coating or a doped diamond-like coating.10. The method of claim 1 wherein the CVD process gas includesconstituents of a diamond-like coating having a dopant that includessilicon or germanium.
 11. The method of claim 1 further comprisingelectrically supplying a current and splitting the supplied current at arespective selected proportion to each of the plurality of anodes. 12.The method of claim 1, wherein a differential mode is used inestablishing current to each of said anodes.
 13. The method of claim 1,wherein during said method steps to maintain a plasma, also controllingaxial temperature profile along said high aspect ratio workpiece.
 14. Amethod for plasma enhanced chemical vapor deposition (CVD) to aninterior region of a hollow tubular high aspect ratio workpiece, whereinthe interior region of the workpiece is oriented horizontally, themethod comprising coating the interior region as a single section by:providing at least three anodes in axially spaced apart arrangementalong an entire lengthwise axis of an elongated interior region of ahollow tubular workpiece of high aspect ratio greater than or equal toabout thirty to one; introducing a CVD process gas into the interiorregion of the workpiece; introducing a plasma moderating gas into theinterior region of the workpiece, the plasma moderating gas beingintroduced predominantly in an upper half of the interior region of theworkpiece; applying a respective individualized DC or pulsed DC bias toeach of the plurality of anodes to excite the process gas into a plasma;and controlling a pressure of the process gas in the interior region ofthe workpiece to maintain a plasma.